. 201. OPTICAL PROPERTIES OF MINERALS. 291 



5. Chcsnut-brown. The purest brown colour. Ex. Egyp- 

 tian Jasper, a variety of rhombohedral Quartz, mixed 

 with oxide of iron. 



6. Yellowish-brown. Brown with a great deal of yellow. 

 Ex. Iron-flint and common Jasper, both varieties of rhom- 

 bohedral Quartz, mixed with oxide of iron. 



7- Pinchbeck-brown. Yellowish-brown, with a metallic 

 lustre, or with a metallic-pearly lustre. Ex. Several varie- 

 ties of rhombohedral Talc-mica. In this mineral, at least, 

 pinchbeck-brown does not deserve the name of a metallic 

 colour, since it is only superficial, and is changed in the 

 streak into white or grey. 



8. Wood-brown. Brown with yellow and grey. The 

 colour of old, nearly rotten wood. Very distinct in several 

 varieties of hemi-prismatic Augite-spar, called Mountain 

 Wood ; sometimes also in Bituminous Wood. 



9. Liver -brown. Brown with grey and a little green. 

 Ex. Common Jasper, a variety of rhombohedral Quartz, 

 mixed with oxide of iron and clay ; it occurs also in brown 

 Earthy Cobalt, which is oxide of cobalt mixed with clay. 



10. Blackish-brown. Brown with a great deal of black. 

 Ex. Several varieties of black Mineral-resin, and of bitu- 

 minous Mineral-coal, called Brown-coal. 



The mentioned varieties of colours represent as many 

 fixed points, between which there exist in nature a great 

 number of shades or varieties. Such colours are expressed 

 by the indications of those two, with which they agree near- 

 est. It is not necessary, however, that these two colours 

 should be consecutive ones in the above series ; they may 

 even be varieties of different principal colours. If the oc- 

 curring colour differs but little from one of these fixed 

 points, it is said to represent, or to be that colour, only 

 inclining, or passing, into another. 



Colours may be different in their intensity, though be- 

 longing to one and the same variety. Differences of this 

 kind are indicated by the expressions, pale, light, deep, dark, 

 which expressions do not require any further explanation. 



